1. Field of Invention
This invention relates to a special type of switch circuit for power electronics, intended to supply electric power to any type of load. The load may be an electric motor, an electric lamp, an electric loudspeaker or any other type of system that needs a controlled supply of voltage and current. The power level can be from less than a watt to hundreds of kilowatts. The direction of power flow can also be regenerative, i.e. from the load to the power supply. The major purpose of the special switch circuit is to supply controllable electric power with low electric losses in the switch circuit and with low electric noise at the input and output of the switch circuit.
A current comparator compares the measured inductor current with a pre-set reference value of inductor current and reacts when the measured current exceeds the pre-set value.
It is not easy to measure the inductor current in a simple, accurate and economical way. The present invention discloses a method and means to measure the current by means of a measuring winding in the inductor and a very simple integrator circuit.
2. Prior Art
The U.S. Pat. No. 4,947,309 “Method and Means for Controlling a Bridge Circuit” describes a special switch circuit. In the following, “switch circuit” refers to this special switch circuit. It is a classical bridge circuit, consisting of two transistor switches and two free-wheel diodes connected in series between a positive and a negative power bus. The bridge circuit is combined with an LC filter at the output of the transistor bridge plus a control circuit with a special switch sequence for controlling the bridge circuit. The primary purpose of the LC filter is to isolate the bridge current from the load current in such a way that the bridge current can go to zero during each switching cycle, while the load current is constant or approximately constant. Such a load current is typical for all applications with inductive load, for example an electric motor. Thanks to the LC filter and the special switch sequence it is possible to turn on the upper or lower transistor at zero transistor current and essentially zero transistor voltage. This eliminates the high electric stress and the electric losses that occur during turn on of conventional bridge circuits. A secondary purpose of the LC filter is to isolate the switching transients in the bridge circuit from the output of the switch circuit. Thanks to this it is possible to control high electric power with low electric noise at the output of the switch circuit.
The switch circuit is suitable for use in so called “frequency inverters” for control of electric AC motors. Then one switch circuit is needed for every motor phase. Only one switch circuit is described in the following.
The control system for the switch circuit requires a feedback signal representing the inductor current. In the prior art a circuit with operational amplifiers has been used for estimation of the inductor current. The present invention provides a simplified measuring circuit for integrating an induced voltage in a measuring winding on the inductor to obtain a representation of the inductor current. This solution is much simpler, costs much less and is more reliable and accurate. The current measuring circuit is very stable and absolutely free from any source of offset signals. Also, it has no upper frequency limitations.